The present invention is directed to bicycle transmissions and, more particularly, to an internally mounted multi-speed hub transmission for a bicycle.
An internally-mounted multi-speed hub transmission sometimes is mounted to the rear wheel of a bicycle so that the rider can select different gear ratios to vary the pedaling effort. A typical hub transmission includes a hub axle that is mounted to the bicycle frame, a driver rotatably supported to the hub axle for receiving the pedaling force through a sprocket and chain, and a hub shell rotatably supported to the hub axle. A power transmitting mechanism is disposed between the driver and the hub shell for communicating rotational power from the driver to the hub shell through a plurality of power transmission paths, wherein each power transmission path typically produces a unique gear ratio. The power transmitting mechanism ordinarily comprises a planetary gear mechanism including one or more sun gears rotatably supported around the hub axle, a ring gear rotatably supported around the hub axle, a planet gear carrier rotatably supported around the hub axle, and a plurality of planet gears rotatably supported to the planet gear carrier and meshing with the sun gear and the ring gear. The plurality of power transmission paths and the corresponding gear ratios are selected by selectively nonrotatably coupling the various components to each other. For example, one gear ratio may be selected by nonrotatably coupling a sun gear to the hub axle, another gear ratio may be selected by nonrotatably coupling the driver relative to the planet gear carrier, and another gear ratio may be selected by nonrotatably coupling the driver relative to the ring gear. Many such coupling relationships often are possible in a typical hub transmission, thus resulting in a relatively large number of possible gear ratios.
It is sometimes desirable to maximize the number of gear ratios produced by the hub transmission so that the rider has greater flexibility to adjust speed and/or pedaling effort over various terrains. This often requires adding planet gears having different diameters that, in turn, require increasing the size of the planet gear carrier or even adding additional separate planet gear carriers. Planet gear carriers ordinarily must be constructed to withstand the forces generated during acceleration and braking. As a result, conventional planet gear carriers are constructed from steel that is machined and/or subjected to heat treatment. Thus, as the number of available gear ratios increase, the weight of the planet gear carrier, and hence the hub, and the number of machining steps required to accommodate the additional planet gears, becomes excessive. Also, using a single planet gear carrier makes it necessary to replace the entire planet gear carrier if it becomes damaged, and it is more difficult to service the planet gears.